Snowploughs can be single-purpose integral machines, having a plough permanently mounted on a vehicle with an engine, designed specifically for the purpose of clearing snow and ice from a roadway or other surface. Alternatively, a snowplough may be provided as an add-on or accessory which is to be mounted on a general purpose vehicle such as a truck or tractor or a multipurpose implement carrier.
When a snowplough is provided as an accessory, it is typically mounted to the front of the vehicle, but sometimes to the rear. Some ploughs are designed to be slanted permanently to the near-side of the vehicle (i.e. closest to the kerb in normal traffic flow), so that in use, snow is displaced towards the kerb. Other ploughs are designed to be more versatile, with the slant angle being variable or switchable, so that the plough can displace snow and ice sideways either to the left or right, this being also useful in open spaces with no kerbs.
The simplest way of achieving this is to mount the plough on a pivot, so that the blade of the plough can be swung to one side or the other. In other words, the right side edge of the blade can be advanced ahead of the left when the plough is swung in the counter-clockwise direction (when viewed from above) or the left side edge can be advanced ahead of the right when the plough is swung clockwise. In the former configuration, as the plough is driven forward, it displaces snow to the left, and in the latter configuration to the right. For ease of reference the former configuration will be described herein as a left-displacing plough and the latter as a right-displacing plough.
A disadvantage of this simple arrangement is that the pivoting action of the plough, as it swings relative to the vehicle, also results in lateral displacement of the plough. Thus, the leading edge will move laterally across the straight-ahead line of travel of the vehicle in the same direction as the displacement direction of the snow, i.e. the leading right-hand edge moves left when the plough is left-displacing and right when the plough is right-displacing.
FIG. 1 shows such a plough 10 mounted on the front 12 of a vehicle having a body 14 and wheels, of which only the left front wheel 16 and right front wheel 18 are shown. (As the vehicle's direction of travel is down the page, the left and right are reversed in this view). The connection between the vehicle mounting 20 and the plough body 22 takes the form of two triangular plates 24, 26 pivoted together about a pivot point 28.
A plough blade 30 at the front of the plough body 22 clears a path when the plough is lowered and driven forward over the ground in a direction of forward travel 32. The cleared path is defined between a pair of parallel dot-dash lines 34, 36 defined by the path of the right and left edges respectively of the plough in the direction of travel. The tracks of the left and right wheels are shown by dashed lines 38, 40 (for the left wheel 16) and 42, 44 (for the right wheel 18). It can be seen that the wheel tracks lie within the ploughed area, giving the vehicle traction in adverse conditions.
Also seen in FIG. 1 are a pair of castors 46 (which could be replaced by slides) on which the plough body is additionally supported. Not visible in FIG. 1, but of relevance, is a rotatable cylindrical brush which is mounted within the plough body 22 with its axis (indicated by a broken line 48) parallel to and behind the blade 30 of the plough. The plough blade can be selectively raised and the brush lowered to the ground, and the brush rotated so that it can sweep remaining snow on e.g. a second pass over the ploughed ground, or when used over other debris or snow which is not so deep as to require the snowplough. Alternatively the blade can be lowered to contact the ground, raising the brush during ploughing. The operation and raising and lowering of the brush and plough can be independently controlled, or can be linked to a common selection control.
FIG. 2 shows the same plough in use, when slanted in a counter-clockwise direction when viewed from above. In other words, the plough is in a left-displacing configuration. The right-displacing configuration is a mirror image.
Now it can be seen that the pivoting of the plough body 22 about the pivot point 28 results in the lateral displacement or translation of the plough body and in particular the blade 30 across the direction of travel. The result is that the cleared path 34, 36 is no longer aligned with the vehicle, and the right wheel track 42, 44 is no longer within the cleared path's right-hand edge 34, and the effective width of the vehicle has increased. Even if the plough is much wider than the vehicle so that the vehicle remains within the ploughed path, the arrangement involves a considerable swinging of the plough from one side to another as the angle changes.
One solution is to make the plough wide enough that it will extend to cover the tracks of both wheels, regardless of its orientation. However, this results in a plough and vehicle which is significantly less manoeuvrable, especially in narrow spaces, such as along pathways or between aeroplanes at an airport.